Gas-permeable but liquid-impermeable polyolefin microporous films

ABSTRACT

The invention concerns a method for producing a microporous film with strong isotropic tendency permeable to gases and impermeable to aqueous liquids, consisting of at least one layer, comprising the following successive steps: preparing a mixture consisting of a polyolefin matrix containing at least a polymer and at least a mineral and/or organic filler; extruding at least a ply by hot-casting the mixture; pre-stretching the ply with a drag roll; cooling and solidifying the pre-stretched ply, using the drag roll; drawing the solidified ply at sufficient temperature for forming the microporous film. The invention is characterised in that the cooling of the pre-stretched melting ply with the drag roll is partial and limited in controlled manner at a temperature in the temperature range required for its drawing; the drawing of the ply, brought to the temperature required for drawing, by the partial cooling is carried out by traction, at the time of its tangential separation from the drag roll, said roller acting as take-up roller for drawing. Said microporous film can be used on its own or combined with other porous supports for various applications such as corporal hygiene articles, dressings, medical articles, protective clothing, sportswear, insulating coats in the building industry, and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to pliant, flexible and breathable polyolefinmicroporous films formed from at least one layer, which are permeable togases and water vapor but impermeable to aqueous liquids, have highmechanical properties and have a pleasant feel.

The invention also relates to a process for producing, by flowextrusion, polyolefin microporous films consisting of at least onelayer, which films are permeable to gases and water vapor butimpermeable to aqueous liquids, have high mechanical properties and havea pleasant feel.

Finally, the invention relates to the use of the aforementioned films bythemselves or combined in the form of a complex with other poroussupports.

2. Description of Related Art

It has been known for a long time how to produce pliant and flexiblemicroporous films having the ability to breathe, that is to say to bepermeable to gases and to water vapor but impermeable to aqueousliquids, and to combine them with other materials such as macroperforated films, nonwoven fabrics, woven fabrics and papers, theseother materials providing the mechanical properties and the desiredfeel, particularly the textile feel.

The most conventional method for producing these microporous filmsconsists in preparing a compound formed from a matrix of thermoplasticpolymers having elastic properties and from a pulverulent, generallyinorganic, filler, in flow-extruding this compound in the melt into theform of a thick web, in taking this thick web up by means of a metalroll (the surface of which may be provided with patterns in order toemboss the web) having a function of prestretching the molten web andcooling it, then in stretching it at least uniaxially, after havingreheated it to a temperature favorable for this operation, in order toobtain the desired film.

During the stretching (post-cooling) allowing the thick web to pass intothe state of a thin film, the inorganic filler particles dispersedwithin the thermoplastic polymer matrix are at least partially separatedfrom the polymer material by a mechanical effect, creating microchannelswithin and passing through the thickness of the film.

The choice of the hydrophobic character of the polymer matrix and thatof the inorganic filler, and in particular the size of its particles,the quality of the dispersion of the filler within the polymer matrixand the choice of stretch ratio applied to the web in order to form thefilm, are as many factors which favor the creation of thesemicrochannels (the equivalent diameter of which is a few microns) andwhich give the film its porosity with respect to gases and itsimpermeability to aqueous liquids.

The prior art shows that microporous breathable films have formed thesubject of considerable research relating to:

the composition of the polymer matrix and the mineral fillers, such ascalcium carbonate and barium sulfate (U.S. Pat. No. 4,472,328) and theaddition of elastomers (such as polybutadiene) in order to improve thestretchability;

the process for manufacturing the film, and in particular an operationof embossing the flow-extruded and slightly stretched web, in order tocreate variations in film thickness (2 μm to 3 mm) therein over veryshort distances, and also the process of combining it with othermaterials in order to increase the mechanical strength and/or thequality of the feel.

By way of illustration, mention may firstly be made of European PatentEP 232 060 which describes a process for producing a gas-permeableporous film according to the following steps, which comprises:

forming a compound from a polyolefin resin and an inorganic filler;

flow extruding a ribbon from the molten compound;

prestretching, embossing and cooling the molten ribbon on a metal rollin order to give it a rough configuration;

and finally carrying out the stretching proper, uniaxially or biaxially,after reheating it, so as to form the microporous film and impart amacromolecular and crystalline orientation sufficient to give it all thedesired properties.

According to that process:

the polyolefin resin may comprise homopolymers, such as polypropylene,low-density polyethylene, linear low-density polyethylene, high-densitypolyethylene and polybutylene, copolymers, such as ethylene-propylenecopolymer and ethylene-vinyl acetate copolymer, or blends of thesepolymers. Particularly preferred are polypropylene, low-densitypolyethylene, linear low-density polyethylene or high-densitypolyethylene, or a blend of two of more of these polymers;

and the inorganic filler is preferably barium sulfate or calciumcarbonate.

Within this same context, another process may also be cited, this havingformed the subject of European Patent No. EP 283 200. This other processappears to be very similar to that described in the above patent sinceit is carried out according to the same steps but which, however, isdistinguished therefrom:

by the composition of the film formed, which is a blend of a singleethylene/C₄ to C₁₀ alpha-olefin copolymer and of an inorganic filler(calcium carbonate);

and by the pattern of the embossing which appears to be a selection of ageometrical shape (hexagonal, circular or diamond shape).

In either of these processes, the molten compound forming thecomposition of the film is flow-extruded in the form of a hot thick web.This web is almost simultaneously prestretched, cooled and embossed bymeans of a metal cooling and embossing roll which is provided with asurface pattern, bringing the thickness of the completely cooled web toabout 100-150 μm.

Since the embossed web coming from the embossing operation does not yethave the desired characteristics for the microporous film, such as, inparticular, a small thickness and a microporosity giving it the abilityto breathe while being impermeable to aqueous liquids, or mechanicalproperties necessary for its subsequent use, it undergoes, after havingbeen reheated to the suitable temperature (for example by calendering),a stretching operation between two pairs of rollers (forming astretching rig) which rotate at different speeds.

The first pair of rollers constitutes the forwarding pair whichsimultaneously fulfils the roles of delivering the embossed ribbon to bestretched and possibly of being an aid for maintaining the delivered webat the desired temperature for stretching.

The second pair of rollers constitutes the pair for the actualstretching of the reheated embossed ribbon, by applying the desiredstretch ratio to it, this ratio being given by the ratio of the linearspeed delivered by the pair of stretching rollers to that delivered bythe pair of forwarding rollers.

After the stretching operation, the film made according to the processmay undergo a heat treatment, generally under tension in order tocontrol its shrinkage.

Thus, it is obvious that the prestretched molten web entering theembossing zone is consequently and almost fully cooled therein and thatthis embossed web has to be reheated to a temperature sufficient to beable to be correctly stretched in the appropriate stretching zone andconverted as far as possible into a film having the expected properties.

However, when the web coming from the flow extrusion is prestretched,cooled and embossed, the constituent material of the web, which isinitially in an almost isotropic state, undergoes internal physicalmodifications during these steps, particularly macromoleculardeformations and/or a macromolecular orientation and/or change in thecrystallinity, these being frozen-in during the cooling.

Although the web coming from the embossing/cooling is reheated in orderto undergo in the best possible state the actual stretching step(between the two pairs of rollers), this reheating appears always to beinsufficient to completely release the material of the web from itsinternal state frozen by the cooling. This is why the final stretchingof the embossed web takes place on an already stressed material, whichadds further stresses to those existing at the time of stretching. Thisaccumulation of internal stresses is manifested by the amount ofshrinkage by heating the film leaving the stretching unit, which oftenrequires post-stretching thermal stabilization in order to at leastpartially remove these stresses.

Consequently, the film coming from the prestretching, cooling, embossingand then stretching steps according to the prior art cannot be in asufficiently isotropic physical state as is desirable even if it has, byvirtue of these steps, many desirable characteristics.

SUMMARY OF THE INVENTION

This why:

a first object of the invention is to provide an improved process formanufacturing a microporous film which is permeable to gases and towater vapor but impermeable to aqueous liquids, is at least a monolayerand is polyolefin-based, another object of the invention is to provide aprocess for manufacturing a microporous film, which is at least amonolayer, having a physical state with a high isotropic tendency afterstretching the cooled web, that is to say an essentially amorphous andhomogeneous physical state;

another object of the invention is to have a microporous film, which isat least a monolayer, having satisfactory permeability to gases andimpermeability to aqueous liquids, formed by at least uniaxiallystretching at least one web prepared from a compound based on at leastone polyolefin polymer and at least one mineral and/or organic filler.

Consequently, the invention relates both to an improved process forproducing an at least monolayer microporous film permeable to gases butimpermeable to aqueous liquids, and to the film itself.

According to the invention, the process for producing a microporous filmhaving a high isotropic tendency, permeable to gases but impermeable toaqueous liquids, being composed of at least one layer, comprising thefollowing successive steps:

preparing a compound composed of a polyolefin matrix comprising at leastone polymer and at least one mineral and/or organic filler;

extruding a web comprising at least one layer by melt flow of thecompound;

prestretching the web by a drive roll;

cooling and solidifying the prestretched web, by means of the driveroll;

stretching the web, at the temperature suitable for forming themicroporous film, is characterized in that:

the cooling of the prestretched molten web by the drive roll is partialand limited in a controlled manner to a temperature lying within therange of temperatures necessary for stretching it;

the web taken to the necessary temperature by the partial cooling isstretched by pulling it at the moment when its tangential separationfrom said drive roll takes place, said roll acting as a forwardingroller for the stretching operation.

According to the invention, the microporous film, permeable to gases butimpermeable to aqueous media, is characterized by an isotropic,homogeneous and partly amorphous physical state.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically illustrates the extrusion line for processing thecomposition comprising polyolefin and filler.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Thus, according to the invention, the web flow-extruded into at leastone layer and prestretched is only partially cooled, and in a controlledmanner, by the drive roll in order to be taken to a temperature lyingwithin the desired temperature range for stretching said web and givingit the most complete isotropy characteristics, having fewer internalstresses, whereas according to the abovementioned prior art the extrudedand prestretched web is cooled without any precaution and without anylimit by passing it over the cooling and embossing roll and consequentlyhas to be reheated to the temperature necessary for stretching it, inorder to be able to be stretched: the stretched film resulting from theprior art is only partially isotropic, as revealed by its highshrinkability.

This is why, in the process according to the invention, the isotropiccharacteristics of the extruded and prestretched web are basicallyundisturbed by the partial cooling carried out in a controlled manner,but they are fully available at the moment when the web reaches thestretching zone in order to undergo therein this operation which takesplace smoothly, generally with a stretch ratio of less than that used inthe prior art, giving the film obtained all the desired physical andmechanical properties, and in particular a highly isotropic state.

Thus, in contrast to the process of the invention and according to theprior art, it seems that:

the fact of uncontrollably cooling the prestretched extruded web on thedrive roll causes the internal characteristics obtained during theprestretching to be fixed;

the fact of reheating said web, for the stretching operation, after ithas been uncontrollably cooled does not allow the film obtained to havean internal state with an isotropic tendency close to that of the postextrusion web.

Composition used for Producing the Film

The polyolefin microporous film according to the invention, permeable togases and water vapor but impermeable to aqueous liquids, is composed ofa polymer matrix comprising at least one polyolefin polymer and at leastone mineral and/or organic filler.

The polyolefin microporous film according to the invention may comprisea single layer, called main layer, or be formed from several layersjoined together, it being possible for each layer to be produced from aparticular composition which is specific to it.

When the microporous film according to the invention is formed from asingle layer, obtained by stretching an extruded web having a highfiller content, this stretched layer must exhibit excellent mechanicalproperties, particularly a very high tensile strength, in order towithstand, without any damage, the stresses to which it is subjected inits various applications. This is why the polyolefin matrix isessentially composed of at least one polymer meeting this requirementfor excellent mechanical strength, which, for this reason, requires arather high density leading de facto to a high melting point.

However, this layer, within which certain internal stresses aregenerated by the stretching operation, may have certain drawbacks duringthe subsequent operations, such as:

heat-welding of the film to itself or to other materials, which weldsmay be produced over the entire surface of the film or only at pointsdistributed over the surface of said film;

or the application of hot-melt adhesives in order to create complexstructures with other flat products, such as nonwoven fabrics; duringwhich the internal stresses relax due to the effect of the heat causingthe film to shrink.

This is why it may be advantageous for the microporous film according tothe invention to be formed from several layers, a main layer having ahigh mechanical strength (like that mentioned in the case of a monolayerfilm) combined with at least one other so-called specific layer, thepolyolefin matrix of which may be formed not only from the polymer orpolymers used in the formation of the main layer and secondary layersbut also from at least one specific olefin polymer and/or copolymer dueto the effect imparted to the specific layer containing it, such as:

a low density and consequently a lower melting point, allowing, forexample, better adhesion to the cooling roll, very high stretch ratios,great ease of being welded to itself or to other materials, easyapplication of hot-melt adhesives, a great deal of freedom for creatingcomplex structures by combining with other materials, without causingthe main layer of the microporous film to shrink;

a higher density and a higher melting point in order to createaffinities with other associated materials (for example nonwovenfabrics), by hot calendering for example;

modification of the polarity of said specific layer with respect to themain layer;

modification to the surface finish in order to create nonslip,antiblocking or antistatic effects in said specific layers, particularlywhen they are in the external position;

a combination of various olefin polymers and/or copolymers in order tocreate a multilayer film, each layer of which has its own function byvirtue of its own composition.

If the polyolefin microporous film comprises a single so-called mainlayer, this layer is formulated in such a way that it provides the filmproduced with the abovementioned essential and optimum mechanicalproperties.

In the case of a monolayer film, the polyolefin matrix comprises one ormore olefin polymers and/or copolymers obtained by polymerization in thepresence of catalysts of the Ziegler-Natta type or of the metallocenetype, used by themselves or as a mixture, which are chosen from thegroup consisting of plastomeric polyolefins such asethylene/alpha-olefin copolymers, homopolymer polyethylenes (radicallow-density polyethylenes) or ethylene/alpha-olefin copolymers(high-density linear, low-density linear and very low-density linearcopolymers), ethyl/vinyl acetate (EVA), ethylene/methyl acrylate (EMA),ethylene/ethyl acrylate (EEA) and ethylene/acrylic acid (EAA)copolymers, and ionomers, homopolymer polypropylenes, metallocenepolypropylenes and propylene/ethylene and propylene/alpha-olefincopolymers.

The (nonpolar) polyolefins used in the composition of the matrix have adensity which lies within the 0.865 to 0.965 g/cm³ range and preferablywithin the 0.900 to 0.935 g/cm³ range.

The filler of mineral origin used in the composition of the filmaccording to the invention may be chosen from the group consisting ofcalcium carbonate, talc, a clay, kaolin, silica, diatomaceous earth,magnesium carbonate, barium carbonate, magnesium sulfate, bariumsulfate, calcium sulfate, aluminum hydroxide, magnesium hydroxide,calcium oxide, magnesium oxide, zinc oxide, titanium oxide, alumina,mica, glass powder, a zeolite or other materials, and preferably calciumcarbonate nd barium sulfate, used by themselves or as a mixture.

The filler of organic origin used in the composition of the filmaccording to the invention may be chosen, for example, from the groupconsisting of cellulose powders, powders of polymers such aspolyurethane, epoxy, ABS, polyfluorinated polymers, polyamides,polyesters or other polymers, some of these powders being prepared bycryogenic grinding, used by themselves or as a mixture.

The size of the mineral and/or organic filler particles is generallychosen to be less than 40 μm and preferably between 0.5 and 10 μm.

The amount of filler used in the preparation of the composition of thefilm is generally between 50 and 500 parts by weight and preferably from80 to 400 parts by weight per 100 parts by weight of the polyolefinmatrix, it being understood that outside these extreme limits theporosity of the film may be insufficient if there is not enough fillerand the stiffness of the film may be too great if there is an excess ofsaid filler, causing, in the latter case, a difficult stretchingoperation.

Before introducing it into the polyolefin matrix, it may be advantageousfor the mineral filler to undergo a surface treatment by means of awell-known treatment agent chosen from the group of fatty acids or theirsalts, silicone oils and silanes, this treatment favoring the dispersionof the mineral filler within said matrix.

However, it may also be advantageous to introduce, into the polyolefinmatrix, at the same time as the mineral and/or organic filler, adispersing agent (in a small amount) having the property of favoring thedispersion of the filler within the polyolefin matrix, of enabling theamount of filler in the said matrix to be increased and of preventingthe formation of agglomerates of filler particles in the flow-extrudedweb and then in the film obtained by stretching. Such agglomerates maybe the cause of instantaneous defects (tears) or subsequent defects(local macroporosity and liquid permeability) which are to the detrimentof the quality of the microporous film. Such an agent, used according tothe invention, may satisfy the following general formula, which includesat least one free-acid functional group:

R-(P)_(x)-(Q)_(y)-Z

in which (P) represents ethylene oxide, (Q) represents propylene oxide,x and y taking values such that their sum lies within the 0 to 24 range,and R is a saturated or unsaturated, linear or nonlinear alkyl radical,an aryl radical, an alkyl-aryl radical, a saturated or unsaturatedheterocycle, containing from 5 to 28 carbon atoms and preferably from 8to 24 carbon atoms, or else a steroid radical. Said radical R may or maynot also be branched and/or comprise one or more functional groups ofthe halogen, —OH, —COOH, —COOR, —NO₂, —NH₂, —CONH₂, —CN, sulfonic,sulfuric, phosphonic or phosphoric type and Z may be one of thecarboxylic, sulfonic, sulfuric, phosphonic and phosphoric functionalgroups.

If Z comprises several acid functional groups, at least one of them isfree, the others possibly being salified or esterified by means of analcohol of formula R′—OH in which the radical R′ may be a carbon chaincontaining 1 to 4 carbon atoms or one of the radicals belonging to thegroup defined above for R, it being possible for the radical R′ to beidentical to the radical R.

Thus, agents for treating the surface of the mineral and/or organicfiller and/or dispersing agents for said mineral and/or organic fillerwithin the polyolefin matrix help to obtain the best dispersion of thefiller in the polyolefin matrix and consequently to increase thestretchability of the film, by further improving its finenesscharacteristics (reduced final thickness) and its microporosity,resulting in a more breathable film.

The compound formed by the polyolefin matrix and the filler may alsoreceive other active agents such as lubricants, pigments, dyes, heatstabilizers, photochemical stabilizers, fire retardants, plasticizersand antistatic agents. This compound may result, in particular, and in apreferred manner, in the production of a bactericidal and acaricidalmicroporous film by introducing (into said compound) suitable biocides.

If the polyolefin microporous film comprises several layers, thecomposition of the main layer is the same as that mentioned in the caseof a monolayer breathable microporous film. The other layer or layers,called specific layers, combined with the main layer are produced bymeans of particular compositions which differentiate them from the mainlayer and which may also differentiate them among themselves in the caseof a multilayer film, each specific layer possibly having a particularfunction.

The polyolefin matrix of the specific layers is composed of at least oneolefin polymer and/or copolymer which is specific owing to the effectthat it imparts to the layer containing it, and more specificallypolyolefins of very low density (0.865 to 0.915 g/cm³) and/or of lowmelting point (from 60 to 100° C.), polyolefins of higher density (from0.920 to 0.965 g/cm³) and/or of high melting point (from 100 to 160°C.), polyolefins having a polar tendency, particularly ethylene/vinylacetate, ethylene/methyl acrylate, ethylene/ethyl acrylate,ethylene/acrylic acid copolymers, ionomers, and polyolefins modifyingthe surface finish of the layer, in particular in order to allow goodadhesion to polar products such as cellulose (paper) fibers, polyesterfibers, viscose fibers and other fibers.

However, the polyolefin matrix of the specific layers may also comprisethe olefin polymer(s) and/or copolymer(s) used in said main layer.

The polyolefin matrix of each specific layer is composed of 5% to 100%by weight, and preferably of 15% to 100% by weight, with respect to thetotal mass of said matrix of at least one specific olefin polymer and/orcopolymer and of at most 95%, and preferably 85%, of at least one olefinpolymer and/or copolymer used in the composition of the matrix of themain layer.

The layers other than the main layer may also contain at least onemineral and/or organic filler chosen from the abovementioned groups andin amounts chosen within the ranges also already mentioned in the caseof the main layer.

The compound according to the invention, comprising a polyolefin matrixand a mineral and/or organic filler, may be prepared according to knownmethods, it being understood that the means used for applying thesemethods do indeed result in the production, by flow extrusion, of a meltin which the mineral and/or organic filler is correctly dispersed, thatis to say is free of agglomerates.

One of the methods may consist in using the various constituents in amulti-feed twin-screw extruder and in flow-extruding the melt. Anothermethod may consist in preparing, by extrusion, granules of a masterbatchby dispersing the filler in a fraction of the polymers constituting thepolyolefin matrix and then in diluting this masterbatch in the remainingfraction of the polyolefin matrix, or by using a masterbatch composed ofthe composition, and in flow-extruding the melt.

Process for Producing the Film

Once each compound needed for producing each layer (consisting of thepolyolefin matrix and the filler according to the layer to be produced)has been prepared, these compounds may be melt-processed in order toproduce a breathable microporous film formed from one or more layers.

According to the process of the invention for producing a microporousfilm, comprising at least one layer, the melt is hot flow-extrudedthrough a flat die in order to form a molten web by means of thepolyolefin matrix, with a thickness of between 300 and 2000 μm onexiting the die.

The temperature of the die and of the molten web at the moment ofextrusion is between 170° C. and 270° C.

The thick web formed is prestretched by a drive roll, having a generallymetallic surface, combined with a back-up roll having, for example, arubberized surface, or with an air knife, by subjecting it to astretching operation, the stretch ratio of which is generally between 4and 25, and preferably between 5 and 10, between the point where itleaves the extrusion die and the point where it comes into contact withthe drive roll.

According to the invention, the web prestretched by the drive roll ispartially cooled thereon, by coming into contact with it, down to thetemperature necessary for its subsequent stretching, and subsequentlysolidified, in a controlled manner. To do this, the temperature of theroll is controlled in such a way that the temperature of the web incontact with it is brought, by controlled cooling, to the temperaturenecessary for stretching it, i.e. brought into a range of temperaturessuitable for carrying out the stretching of the web under the bestthermal conditions: the temperature given to the web in contact with thedrive roll is chosen within the 40° C. to 165° C. range and preferablywithin the 55° C. to 135° C. range.

The surface of the roll for driving the web may be a smooth surface ofthe shiny or matt type.

However, to avoid any risk of the prestretched web slipping on the driveand cooling roll, the surface of said roll may be formed from astructure having a surface with a high friction coefficient, either bythe material used for said surface or by creating, on this surface, aregular or irregular, or even random, surface structure, such as, forexample, a sand-blasted, satin-finish, speckled or other appearance.

When prestretching the web, cooling it in a controlled manner to adefined temperature and partially solidifying it, said web may alsoundergo an embossing operation by means of said roll. In the lattercase, the surface of the drive roll is provided with a regular orirregular pattern, in the form of projections or of indentations, themost frequent shapes of which may be lines in the longitudinal,transverse or diagonal direction with respect to the direction ofadvance of the web, these being parallel or intersecting one another inorder to form geometrical figures, chevrons of greater or lesser flareangle, sinusoids, toothings, or else figures such as polygonal orcircular figures (for example hexagons, diamonds, circles), or figureshaving a random distribution.

The time during which the web is in contact with the drive roll, whichallows the web to be put into the best temperature range for thestretching operation by cooling it in a controlled manner, may becontrolled through the use of a back-up roll placed in the exit zonetangential to said web. This back-up roll makes it possible to fix thelength of the arc over which the web is in contact with the drive roll,that is to say to set the degree of wrap of the roll by the web byimposing on it the instant when the web leaves said roll tangentially.Thus, the web is optimally prepared for undergoing the stretchingoperation under the best conditions.

According to the invention, the prestretched web, taken to the desiredtemperature (for said stretching) in contact with the drive roll bypartial, regulated and controlled cooling, is immediately stretched in asingle step by pulling it using a pair of rollers the instant when theweb, at the suitable temperature, leaves said drive roll tangentially.

Unlike the conventional stretching unit used in the prior art, whichcomprises two pairs of rollers rotating at different speeds, one forforwarding the web and the other for stretching it, the roll for drivingthe web, which also prestretches it and partially cools it in aregulated and controlled manner, also fulfils the function of aforwarding roll delivering the web to be stretched when it has beentaken to the temperature most favorable for this operation.

After the web to be stretched has left the drive roll tangentially, itis taken up by a pair of rollers, placed downstream of said roll, whichconstitutes the pair of rollers for the actual stretching of thedelivered web, by applying the desired stretch ratio thereto. This pairof stretching rollers, placed downstream of the drive roll, is heated toa temperature of between 40° C. and 165° C. and preferably between 55°C. and 135° C.: this temperature may be identical to or different fromthat of the drive roll. The stretch ratio according to the invention,given by the ratio of the linear speeds delivered by the pair ofstretching rollers and the roll for driving (prestretching, cooling andforwarding) the web and taken to the temperature suitable for thestretching operation, is chosen within the 1.5 to 10 range andpreferably chosen within the 2 to 6 range.

However, it is also possible, according to the invention, for the web(which is prestretched), taken to the desired temperature by partial,regulated and controlled cooling in contact with the drive roll, to bestretched in several successive steps. In this case, the firststretching step, called tangential stretching, is carried outimmediately (as mentioned above) by pulling it using a pair of rollersplaced downstream of the drive roll where the web (which is at thesuitable stretching temperature of between 40° C. and 165° C.) leavessaid drive roll tangentially. In this first step, the web is stretchedto a fraction of the desired total stretch: this fraction lies between80% and 20% of the total stretch envisioned.

After this first stretching step, the partially stretched film mayundergo relaxation by passing over a suitable roll (called relaxationroll), the temperature of which is chosen within the 125° C. to 20° C.range.

After leaving the relaxation roll, the relaxed film is reheated andtaken to the desired temperature by passing over a reheating roll, inorder to continue and/or complete the stretching operation: thetemperature to which the partially stretched film is reheated during thefirst step is chosen within the 40° C. to 165° C. range.

After leaving the reheating roll, the partially stretched film, taken tothe desired temperature for continuing the stretching operation, entersa conventional stretching zone which comprises two pairs of rollersrotating at different speeds, one for forwarding the web and the otherfor stretching it: the stretching of the film may be completed in thisconventional stretching zone.

If the web is stretched in two successive steps, that fraction of thestretch introduced in the second step lies between 20% and 80% of thetotal stretch envisioned.

If the web is stretched successively in three or more steps, thatfraction of the stretch introduced in the first step lies between 20%and 70% of the total stretch envisioned, that fraction of the stretchintroduced in the second step lies between 15% and 70% of the totalstretch envisioned and that fraction of the stretch introduced in theother steps, including the final stretching step (by sharing) liesbetween 15% and 70% of the total stretch envisioned.

When the number of stretching steps is greater than two, the sameabovementioned means, for relaxing the film, reheating the film,stretching the film and relaxing/cooling the film at the end, are againplaced downstream of the second stretching step, which means arerepeated as many times as there are stretching steps.

The microporous film after the stretching operation is completely cooledby a suitable relaxation and cooling station formed, for example, by oneor more rolls in cascade, these decreasing the temperature down to thedesired temperature for storing the film.

The stretched film, beyond the zone for stretching the web and forconverting it into a microporous film, may undergo a heat treatmentallowing the tensile stresses induced in the film throughout themanufacturing process to be relaxed. However, it will be recalled thatsince one of the objects of the invention is to produce a microporousfilm (having at least one layer) in as isotropic a physical state aspossible after stretching the web, the abovementioned internal tensilestresses are smaller (in the film according to the invention) than thosegenerated in the films according to the prior art:

as, according to the invention, the prestretched web is taken to itsstretching temperature by the drive roll,

whereas, according to the prior art, the prestretched web isuncontrollably cooled by the drive roll to a temperature below thatnecessary for stretching it and then subsequently reheated to thestretching temperature in order to be able to be stretched.

The stretching method used according to the invention is much gentlerthan that used in the prior art.

In the same way that the extruded web may undergo an embossing operationby means of the drive roll, the film may undergo an embossing operationin various zones of the process according to the invention.

If said process comprises only a single stretching step (calledtangential stretching step), the film may undergo an embossing operationin one or more subsequent zones of said process:

a) in the tangential stretching zone, using the pair of rollers forstretching the web which is placed downstream of said drive roll, as anembossing means;

d) in the zone for relaxing the stretched film, using at least one ofthe relaxation rolls as embossing means.

If said process comprises several stretching steps, the film may undergoan embossing operation in the next zone and/or zones of said process:

a) in the tangential stretching zone using, as previously, the pair ofrollers for stretching the web, placed downstream of said drive roll, asembossing means;

b) in the reheating zone which precedes the second stretching step,using the reheating roll as embossing means;

c) in the second stretching zone, where the stretching of the film fromthe tangential stretching step is continued, comprising two pairs ofrollers rotating at different speeds, one of the pairs being forforwarding and rotating at a speed close to the reheating roll, theother for stretching and rotating at a speed greater than the pair forforwarding the film to be stretched, using one or both pairs of rollersas embossing means;

d) in the zone for relaxing the stretched film, using at least one ofthe relaxation rolls as embossing means.

Just as the web may be embossed before the stretching operation, thefilm may be embossed in the stretching, relaxation, reheating andrelaxation/cooling zones, by means of the surface of at least one of therollers of the stretching and/or forwarding pairs, of the relaxationand/or reheating rolls and/or of the relaxation/cooling rolls: thesurface of the roller or rollers and/or of the roll or rolls intendedfor embossing the film is provided with a regular or irregular pattern,in the form of projections and/or indentations, the most frequent shapesof which may, for example, be lines in the longitudinal, transverse ordiagonal direction with respect to the direction of advance of the film,these being parallel or intersecting one another in order to formgeometrical figures, chevrons of greater or lesser flare angle,sinusoids, toothings or else figures such as polygonal or circularfigures (in the form of a hexagon, diamond or circle) or figures havinga random distribution. If the embossing is desired for the appearanceand final qualities of the film, this embossing may be carried out oneor more times in the process according to the invention.

When the embossing is carried out only once, this may be done on the webbefore it is stretched by means of the drive roll (zone a)) or on thefilm in one of the abovementioned zones b) or c) or d).

When the embossing is carried out several times, this may be done bycombining, for example:

the embossing of the web (zone a)) with the embossing of the film in atleast one of the zones b), c), d);

the embossing of the single film by combining at least two of th eembossing zones b), c), d).

Likewise, the microporous film according to the invention undergoingstretching in the longitudinal direction may also undergo, beyond thisfirst stretching, another, but transverse, stretching. In this case, thetransverse stretching is carried out by means of known devices with astretch ratio generally between 1.1 and 10 and preferably between 1.1and 4.

Finally, after these various steps, carried out within the context ofthe process according to the invention, the microporous stretched filmis stored continuously on a suitable support at a linear winding speedof 0.5% to 25% less than the linear speed of the film coming from theprevious step, in order to allow it to relax, particularly from thelongitudinal stretching step when the heat treatment step is not carriedout.

Thus, the process according to the invention results in a reduction inthe stretching forces by carrying out the stretching at the mostfavorable temperature without having to make it undergo uncontrolledcooling followed by considerable reheating in order to meet the thermalconditions for the stretching operation.

When the process according to the invention relates to the manufactureof a monolayer microporous film, that is to say one formed from just themain layer having a high mechanical strength, this film is produced bymeans of a single extruder feeding the flow-extrusion flat die.

However, when the process according to the invention relates to themanufacture of a multilayer microporous film, the film may be producedby means of several extruders, i.e. as many extruders as there arecompositions of layers, which feed a flat die provided with acoextrusion device.

The molten web, formed from several flow-coextruded layers, thenundergoes the same steps as those mentioned above for forming amonolayer microporous film, i.e.:

prestretching by the drive roll;

partial and limited cooling in a controlled manner of the prestretchedweb by taking it, by means of the drive roll, to a temperature lyingwithin the desired temperature range for stretching it;

stretching the web by pulling on it at the moment when it leaves saiddrive roll tangentially, possibly controlled by a downstream back-uproll, this roll also acting as a roller for forwarding the web in orderto stretch it.

To demonstrate the effect of the invention on the microporous filmsobtained according to the process, an orientation index for determiningthe physical state of the film or its tendency toward isotropy wasestablished. This index is formed by the ratio of the values of themechanical properties of the films measured in the machine direction tothose measured in the transverse direction (MD/TD). If a film isabsolutely isotropic, this ratio is equal to 1. If a film is completelyoriented in the machine direction, that is to say a nonisotropic film,this ratio tends toward infinity.

When the mechanical properties of the films, such as the tensilestrength, elongation as break and the loads at 5%, 10%, 15% and 25%elongation, are measured, the index of the film according to theinvention is always between the value 1 and the value of the index ofthe film produced according to the prior art.

More precisely, and for the aforementioned mechanical properties (exceptfor the elongation at break), the index of the film according to theinvention is from 1.4 to 2 times smaller than the index of the filmaccording to the prior art and tends toward the value 1, showing itsability to be more isotropic than films according to the prior art.

The microporous film according to the invention can be used by itself orcombined with other porous supports such as, for example, nonwovenfabrics, woven fabrics, knits, net-type meshes and paper, for variousapplications such as body hygiene articles, bandages, medical articles,protective clothing, sports clothing, insulating coverings in thebuilding industry, and other applications.

In order to better understand the invention, the process is describedaccording to the single figure for the particular case of the processwith two-step stretching, this in no way, having any limiting effect onits scope.

According to this figure, the compound, consisting of the polyolefinmatrix and the filler, is melt processed by extruding it through theflat die (1) at the temperature specific to said compound.

The thick web (2) formed is prestretched by the drive roll (3) having amatt metal surface combined with a back-up roll (4) with a rubberizedsurface, to a suitable and chosen amount of stretch. The surface of thedrive roll may be provided with a regular or irregular pattern, in theform of indentations or projections, in order to allow the web to beembossed. The web (2) prestretched by the drive roll is partially cooledthereon, by being in contact with it, down to the temperature necessaryfor the first step of a two-step stretching operation. The time duringwhich the web (5) is in contact with the drive roll (3) is controlled bythe back-up roll (6) which, depending on its position, increases ordecreases the length of the arc over which the roll (3) is wrapped bythe web (5): the point where the web (5) leaves tangentially iscontrolled by the back-up roll (6).

As soon as the web (5) to be stretched leaves the drive roll (3)tangentially, it is taken up by a pair of rollers (8) and (9), placeddownstream of said roll (3), which constitutes the pair of stretchingrollers tangential to the first, partial stretching step, by applyingthe desired stretch ratio to it. Thus, the web (5) is stretched into thefilm (7) between the drive roll (3) and the pair of rollers (8) and (9).The cylindrical surface of at least one of the rollers (8) and (9) maybe provided with patterns, like those mentioned above, in order to allowthe film to be embossed.

After leaving this first stretching step, the partially stretched film(10) undergoes relaxation by passing over the roll (11) whosetemperature is regulated within the abovementioned range.

After leaving the relaxation roll, the relaxed film (10) is reheated onthe roll (12) and, in contact with it, taken to the desired temperaturefor completing its stretching in a second step. The temperature of theroll (12) is set to a value chosen within the abovementioned range. Inaddition, the cylindrical surface of the roll (12) may be provided withpatterns, as mentioned above, in order to allow the film to be embossedas it is being reheated. In the case of the embossing of the relaxed andreheated film, the roll may be provided with a back-up roll with arubberized surface (not shown).

The partially stretched film (13), taken to the desired temperature,enters a conventional stretching zone (second stretching step) whichcomprises two pairs of rollers rotating at different speeds, one pair[the rollers (14) and (15)] for forwarding the film and the other pair[the rollers (17) and (18)] for stretching it: the stretching of thefilm (13) into the film (16) is completed between these two pairs ofrollers. The cylindrical surface of at least one of the rollers (14) and(15) and/or of the rollers (17) and (18) may be provided with patternsin the form of indentations or of projections (as indicated above) inorder to allow the film to be embossed.

After leaving the pair of rollers (17) and (18) of the second stretchingstep, the stretched film (19) enters a relaxation station comprising, incascade, the rolls (20), (21) and (22) delivering the stretched andpossibly embossed film (23) at a chosen temperature for storing it on areel. The cylindrical surface of the roll (20) may be provided withpatterns, such as those mentioned above, in order to allow the film tobe embossed after the final stretching step.

Thus, the embossing may be carried out either on the web, or on thefilm, or successively on the web and then the film, in as many steps asdeemed to be desirable for the final quality of the film.

The invention will be more clearly understood by means of the examplesgiven below, the physical properties of the films obtained having beendetermined according to the ASTM standards indicated.

EXAMPLE 1

In order to determine the orientation index of the physical state(mentioned above) of films manufactured according to the invention andaccording to the prior art, three films were produced (with referencenumber 3 for the invention and reference numbers 1 and 2 for the priorart) on the same flat die extrusion line of a pilot plant whichcomprised:

a 45 mm diameter COLLIN extruder (300 mm die with a 0.6 mm opening);

a drive-cooling-embossing roll (125 mm in diameter) and a rubber back-uproll (125 mm in diameter).

The composition of the three films was the same: it consisted of acompound based on a polyolefin matrix and a CaCO₃ filler (having adiameter of between 1 and 5 μm) in an amount of 110 parts by weight ofthe filler per 100 parts by weight of the matrix.

The polyolefin matrix was formed from:

a 50/50% blend of an LDPE 42.6% homopolymer and an ethylene/alpha-olefin copolymer, constituting the polyolefin part of the masterbatchsold under the reference AMPACET 100196 by Ampacet Europe; anethylene/octene copolymer 57.4% (plastomer) sold under the brandAFFINITY PL 1845 by Dow Chemical; density: 0.910 g/cm³; melt flow index:2.5 g/10 min (ASTM 1238)

The webs (references 1, 2 and 3) obtained by flow extrusion of thecompound and then prestretched by means of the drive-cooling-embossingroll had thicknesses at the entrance of the roll of 80 to 85 μm. Thefilm (reference 3) according to the invention was partially cooled onthe drive-cooling-embossing roll to a temperature of 60° C., desired forstretching it, and immediately stretched at this temperature will astretch ratio of 4 as soon as it leaves said roll tangentially,determined by the presence of a back-up roll.

The films (references 1 and 2) were cooled on thedrive-cooling-embossing roll to a temperature of 30° C. and then, afterleaving the cooling zone, were reheated to 60° C. in the case ofreference film 1 and 80° C. in the case of reference film 2, and theywere then stretched at these temperatures with a stretch ratio of 4 on asuitable stretching unit. All the physical properties of the films, andthe corresponding index, are given in Table 1 below.

TABLE 1 Values measured on Mea- the films sure- (x) (xx) (xxx) ment Ref.1 Ref. 2 Ref. 3 Property unit Standards 60° C. 80° C. 60° C. Grammageg/m² 34 32.8 34 Thickness μm ACE 26.1 25.2 26 (calculated) Embossed μmASTM E 252 43 38 40.6 thickness Load at break N/inch ASTM D 882 MD 30.327.4 24.6 TD 2.4 2.4 3.1 MD/TD 12.6 11.4 7.9 Elongation at % ASTM D 882break MD 70 63 77 TD 368 319 348 MD/TD 0.19 0.20 0.22 Load at 5% N/inchASTM D882 elongation MD 6.28 6.41 6.56 TD 0.82 1 1.69 MD/TD 7.7 6.4 3.9Load at 10% N/inch ASTM D 882 elongation MD 12.05 11.86 11.28 TD 1.31.42 1.89 MD/TD 9.3 8.4 6.0 Load at 15% N/inch ASTM D 882 elongation MD16.78 16.21 14.75 TD 1.53 1.57 1.98 MD/TD 11.0 10.3 7.4 Load at 25%N/inch ASTM D 882 elongation MD 22.85 21.57 18.53 TD 1.66 1.68 2.07MD/TD 13.8 12.8 9.0 Breathability second ACE 5 4.5 7 Water-vapor G/m²/ASTM E-96 5700 6000 4500 permeability at 24 h 38° C., 90% RH Resistanceto m EDANA 1.43 1.45 1.27 water column 120-1-80 (x) according to theprior art (xx) according to the prior art (xxx) according to theinvention.

EXAMPLE 2

A three-layer polyolefin microporous film, permeable to gases butimpermeable to liquids, was produced by the process of the invention bymeans of an industrial plant, the specific layers being layers of lowmelting point in order to make it easier for said film to besubsequently joined, hot, to a nonwoven fabric.

The main (internal) layer of the finished film had a thickness of 20 μm,while the specific (external) layers each had a thickness of 2.5 μm.

In order to produce such a film, the composition of the main layer wasthe same as that described in Example 1.

As regards the two specific (external) layers of the same composition,they were also composed of a polyolefin matrix, a mineral filler andvarious specific agents, the mineral filler being present in thecompound in an amount of 110 parts by weight per 100 parts by weight ofthe olefin matrix.

The polyolefin matrix of the specific layers was formed from 3 polymersin the following percentages by weight:

a 50/50% blend of an LDPE 35.8% homopolymer and an ethylene/alpha-olefincopolymer, constituting the polyolefin part of the masterbatch soldunder the reference AMPACET 100196 by Ampacet Europe; a very low-density64.2% polyethylene sold under the brand AFFINITY KC 8852 by DowChemical; *density: 0.875 g/cm³; *melt flow index: 3 g/10 min

The mineral filler was composed of CaCO₃ the particles of which had adiameter of between 1 μm and 5 μm. Various agents, such as a biocide (inparticular an acaricide), a lubricant, a heat stabilizer, etc., wereintroduced into the matrix in the usual amounts.

Each dry blend intended for each layer was extruded in an extruderspecific to each layer.

The three extruders working in parallel fed a flat die provided with acoextrusion device.

The temperature of the flat die and of the flow-extruded three-layer webwas about 200° C. and the lip of the open die between 0.3 and 2 mm.

The extruded web, formed from three layers, was taken up between themetal drive roll and a back-up roll provided with a rubber coating.

The three-layer web was thinned down by a factor of about 10.

The temperature of the metal drive:roll was set at 65° C. so that theweb was heated to this same temperature. As in Example 1, the metaldrive roll acted on the molten web by:

the drive function;

an effect of stabilizing the temperature of the prestretched web bymaking it come to the desired temperature for the stretching step bypartial and controlled cooling of the flow-extruded web;

the creation of a rough configuration on the web, with a depth ofbetween 2 μm and 3 mm, by embossing it on the metal roll, the surface ofwhich is provided with projecting patterns;

by another function, namely that of the roller for forwarding theprestretched web and taken to a suitable temperature, delivering saidweb in order to stretch it, as soon as it leaves tangentially.

The actual stretching of the web was carried out in the longitudinaldirection at a temperature of 65° C. with a stretch ratio of 3 to 4.

The microporous film thus obtained did not undergo further treatments:it was slit in line and then wound for the purpose of its applications.

The physical properties of the three-layer film are given in Table 2below:

TABLE 2 Values measured on the films according to the invention Three-Measure- layer Mono- Measured ment coextruded layer properties unitStandard film film Grammage g/m² 33 33 Thickness μm ASTM E 252 25.5 25.6Tensile strength N/25 mm ASTM D 882 machine direction 18.1 18.5transverse direction 2.5 3.7 Elongation at % ASTM D 882 break machinedirection 162 175 transverse direction 367 310 Breathability Second ACE14 16 Resistance to cm EDANA >100 >100 water column 120-1-80 Strength ofthe ASTM D 882 weld produced: at 75° C. peelable 0 at 80° C. welded 0 at100° C. welded peelable at 105° C. welded welded

What is claimed is:
 1. A process for producing a microporous film havinga high isotropic tendency, that is permeable to gases but impermeable toaqueous liquids, said film comprising at least one layer, comprising thefollowing successive steps: (a) forming a mixture comprising apolyolefin matrix which comprises at least one polymer and at least onemineral and/or organic filler; (b) extruding a web comprising at leastone layer of said mixture by melt flow; (c) prestretching the web uponcontact with a drive roll; (d) cooling and solidifying the prestretchedweb by contact with said drive roll, wherein said cooling of theprestretched web is partial and limited to a temperature within therange permitting further stretching of said web; and (e) furtherstretching the solidified web, at a temperature suitable for forming amicroporous film, wherein said further stretching is achieved by pullingsaid web at the moment when tangential separation from said drive rolloccurs, said roll acting as a forwarding roll for said furtherstretching operation.
 2. The process as claimed in claim 1, wherein thepoint where the web leaves the drive roll tangentially is controlled bya back-up roll, which fixes the arc length over which the web is incontact with said drive roll.
 3. The process as claimed in claim 1,wherein when the film comprises a single layer, or main layer, thepolyolefin matrix is formed from olefin polymers and/or copolymersobtained by polymerization in the presence of catalysts of theZielger-Natta type or of the metallocene type, which are selected fromthe group consisting of ethylene/alpha-olefin copolymers, homopolymerpolyethylenes, radical low-density polyethylenes, or ethyl/vinylacetate, ethylene/methyl acrylate, ethylene/ethyl acrylate andethylene/acrylic acid copolymers, and ionomers, polypropylenes,metallocene polypropylenes and propylene/ethylene andpropylene/alpha-olefin copolymers, and mixtures thereof.
 4. The processas claimed in claim 1, wherein nonpolar polyolefins used in the mixturehave a density which ranges between about 0.865 g/cm³ (3.124×10⁻²lb/in³) and about 0.965 g/cm³ (3.486×10⁻² lb/in ³).
 5. The process asclaimed in claim 4, wherein the nonpolar polyolefins used in the mixturehave a density which ranges between about 0.900 g/cm3 (3.251×10⁻²lb/in³) and 0.935 g/cm3 (3.378×10⁻² lb/in³).
 6. The process as claimedin claim 1, wherein the mineral filler used in the mixture is selectedfrom the group consisting of calcium carbonate, talc, a clay, kaolin,silica, diatomaceous earth, magnesium carbonate, barium carbonate,magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide,magnesium hydroxide, calcium oxide, magnesium oxide, zinc oxide,titanium oxide, alumina, mica, glass powder, a zeolite and mixturesthereof.
 7. The process as claimed in claim 6, wherein the mineralfiller used in the mixture comprises calcium carbonate, barium sulfate,and mixtures thereof.
 8. The process as claimed in claim 6, wherein themineral filler is treated with a treatment agent selected from the groupconsisting of fatty acids or their salts, silicone oils and silanes. 9.The process as claimed in claim 1, wherein the organic filler used inthe mixture is selected from the group of powders consisting ofcellulosic polymers, polyurethanes, epoxies, acrylonitrile butadienestyrene polymer, polyfluorinated polymers, polyamides, polyesters, andmixtures thereof.
 10. The process as claimed in claim 1, wherein thesize of the filler particles is less than about 40 μm (1.575×10⁻³ in).11. The process as claimed in claim 10, wherein the size of the fillerparticles ranges between about 0.5 μm (1.968×10⁻⁵ in) and about 10 μm(3.937×10⁻⁴ in).
 12. The process as claimed in claim 1, wherein theamount of filler used in the mixture ranges between about 50 and about500 parts by weight per 100 parts by weight of the polyolefin matrix.13. The process as claimed in claim 12, wherein the amount of fillerused in the mixture ranges between about 80 and about 400 parts byweight per 100 parts by weight of the polyolefin matrix.
 14. The processas claimed in claim 1, wherein the web is hot flow-extruded through aflat die forming a molten web with a thickness ranging between about 300μm (0.0118 in) and 2000 μm (0.0787 in) on exiting the die.
 15. Theprocess as claimed in claim 14, wherein the dispersing agent has theformula: R-(P)_(x)-(Q)_(y)-Z where Z comprises at least one free-acidfunctional group, selected from the group consisting of carboxylic,sulfonic, sulfuric, phosphonic or phosphoric acid radicals, (P) isethylene oxide, (Q) is propylene oxide, where 0≦(x+y)<24, and R isselected from the group consisting of saturated or unsaturated, linearor nonlinear alkyl radicals, aryl radicals, alkaryl radicals, saturatedor unsaturated heterocycles, containing from about 5 to about 28 carbonatoms, and a steroid radical.
 16. The process as claimed in claim 15,wherein R is selected from the group consisting of saturated orunsaturated, linear or nonlinear alkyl radicals, aryl radicals, alkarylradicals, saturated and unsaturated heterocycles, containing from 8 to24 carbon atoms, or a steroid radical.
 17. The process as claimed inclaim 15, wherein when Z comprises several acid functional groups, atleast one of said acid functional groups is free, the others beingsalified or esterified by an alcohol of formula R′—OH in which R′ isselected from the group consisting of a carbon chain containing fromabout 1 to about 4 carbon atoms or one of the radicals selected from thegroup consisting of saturated or unsaturated, linear or nonlinear alkylradicals, aryl radicals, alkaryl radicals, saturated or unsaturatedheterocycles, containing from 8 to 24 carbon atoms, and a steroidradical.
 18. The process as claimed in claim 15, wherein R comprises atleast one functional group selected from the group consisting ofhalogen, —OH, —COOH, —COOR, —NO₂, —NH₂, —CONH₂, —CN, sulfonic, sulfuric,phosphonic and phosphoric type.
 19. The process as claimed in claim 15,wherein the radical R′ is identical to the radical R.
 20. The process asclaimed in claim 1, wherein the mixture formed by the polyolefin matrixand the filler further comprises lubricants, pigments, dyes, heatstabilizers, photochemical stabilizers, fire retardants, plasticizers,antistatic agents, and biocides.
 21. The process as claimed in claim 1,wherein when the web comprises several layers, the olefin components ofthe mixture in each specific layers comprise at least one olefin polymerand/or copolymer selected from the group consisting of polyolefinshaving a density ranging between about 0.865 g/cm³ (3.124×10⁻² lb/in³)and about 0.915 g/cm³ (3.306×10⁻² lb/in³) and/or a melting point rangingbetween about 60° C. (140° F.) and about 100° C. (212° F.), polyolefinshaving a density ranging between 0.920 g/cm³ (3.324×10⁻² lb/in³) and0.965 g/cm³ (3.486×10⁻² lb/in³) and/or having a melting point rangingbetween about 100° C. (212° F.) and about 160° C. (320° F.), polarpolyolefins, ethylene/vinyl acetate, ethylene/methyl acrylate,ethylene/ethyl acrylate, ethylene/acrylic acid copolymers, ionomers, andpolyolefins modifying a surface finish of the layer.
 22. The process asclaimed in claim 21, wherein the polyolefin component of the mixture ineach specific layer comprises about 5% to about 100% by weight, withrespect to the total mass of said polyolefin component, of at least onespecific olefin polymer and/or copolymer and of at most about 95% of atleast one olefin polymer and/or copolymer used in the matrix of the mainlayer.
 23. The process as claimed in claim 22, wherein the polyolefincomponent of the mixture in each specific layer comprises about 5% toabout 100% by weight, with respect to the total mass of said polyolefincomponent, of at least one specific olefin polymer and/or copolymer andof at most about 85% of at least one olefin polymer and/or copolymerused in the matrix of the main layer.
 24. The process as claimed inclaim 22, wherein the polyolefin component of the mixture in eachspecific layer comprises about 15% to about 100% by weight, with respectto the total mass of said polyolefin component, of at least one specificolefin polymer and/or copolymer and of at most about 95% of at least oneolefin polymer and/or copolymer used in the matrix of the main layer.25. The process as claimed in claim 24, wherein the polyolefin componentof the mixture in each specific layer comprises about 15% to about 100%by weight, with respect to the total mass of said polyolefin component,at least one specific olefin polymer and/or copolymer and of at mostabout 85% of at least one olefin polymer and/or copolymer used in thematrix of the main layer.
 26. The process as claimed in claim 20,wherein one or more of the specific layers comprise: a) a mineral fillerselected from the group consisting of calcium carbonate, talc, a clay,kaolin, silica, diatomaceous earth, magnesium carbonate, bariumcarbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminumhydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, zincoxide, titanium oxide, alumina, mica, glass powder, a zeolite andmixtures thereof, and/or b) an organic filler selected from the group ofpowders consisting of cellulosic, polymers, polyurethanes, epoxies,acrylonitrile butadiene styrene polymer, polyfluorinated polymers,polyamides, polyesters, and mixtures thereof; and c) a polyolefin,wherein the amount of filler used in the mixture ranges between about 50and about 500 parts by weight per 100 parts by weight of the polyolefinmatrix.
 27. The process as claimed in claim 21, wherein the one or morespecific layers comprise: a) a mineral filler selected from the groupconsisting of calcium carbonate, talc, a clay, kaolin, silica,diatomaceous earth, magnesium carbonate, barium carbonate, magnesiumsulfate, barium sulfate, calcium sulfate, aluminum hydroxide, magnesiumhydroxide, calcium oxide, magnesium oxide, zinc oxide, titanium oxide,alumina, mica, glass powder, a zeolite, and mixtures thereof; and/or b)an organic filler selected from the group of powders consisting ofcellulosic, polymers, polyurethanes, epoxies, acrylonitrile butadienestyrene polymer, polyfluorinated polymers, polyamides, polyesters, andmixtures thereof; and c) a polyolefin, wherein the amount of filler usedin the mixture ranges between about 80 and about 400 parts by weight per100 parts by weight of the polyolefin matrix.
 28. The process as claimedin claim 1, wherein the web is hot flow-extruded through a flat dieforming a molten web with a thickness ranging between about 300 μm(0.0118 in) and 2000 μm (0.0787 in) on exiting the die.
 29. The processas claimed in claim 1, wherein a multiplicity of microporous webs areformed, the mixtures corresponding to the various webs are melt extrudedby at least as many extruders as there are webs of different mixture,these extruders feeding a flat die provided with means forflow-coextrusion, forming a molten web of thickness ranging betweenabout 300 μm (0.0118 in) and about 2000 μm (0.0787 in) on exiting thedie.
 30. The process as claimed in claim 1, wherein the temperature ofthe molten web upon extrusion is between about 170° C. (338° F.) andabout 270° C. (518° F.).
 31. The process as claimed in claim 1, whereinthe molten flow-extruded web is prestretched by means of a drive rollwith a stretch ratio within the range about 4/1 to about 25/1.
 32. Theprocess as claimed in claim 31, wherein the molten flow-extruded web isprestretched by means of a drive roll with a stretch ratio within therange about 5/1 to about 10/1.
 33. The process as claimed in claim 1,wherein the prestretched web is partially cooled, in a controlledmanner, by the drive roll, to a temperature permitting furtherstretching the web.
 34. The process as claimed in claim 33, wherein thecontrolled temperature to which the web is partially cooled is withinthe range between about 40° C. (104° F.) and about 165° C. (329° F.).35. The process as claimed in claim 34, wherein the controlledtemperature to which the web is partially cooled is within the rangebetween about 55° C. (131° F.) and about 135° C. (275° F.).
 36. Theprocess as claimed in claim 1, wherein the surface of the drive roll hasa high coefficient of friction, is smooth, and exhibits a shine or mattesurface.
 37. The process as claimed in claim 1, wherein the surface ofthe drive roll is provided with a regular or irregular pattern, in theform of projections or of indentations, in shapes which are lines in thelongitudinal, transverse or diagonal direction with respect to thedirection of advance of the web, these being parallel or intersectingone another in order to form geometrical figures, chevrons of greater orlesser flare angle, sinusoids, toothings or polygonal or circularfigures.
 38. The process as claimed in claim 37, wherein the web,prestretched and cooled in a controlled manner to a defined temperature,is embossed by means of the drive roll.
 39. The process as claimed inclaim 1, wherein the stretching of the web, is carried out by pulling itat the moment said web leaves the drive roll tangentially, in a singlestep at a temperature ranging between about 40° C. (104° F.) and about165° C. (329° F.).
 40. The process as claimed in claim 39, wherein thestretching of the web, is carried out by pulling it at the moment saidweb leaves the drive roll tangentially, in a single step at atemperature ranging between about 55° C. (131° F.) and about 135° C.(275° F.).
 41. The process as claimed in claim 39, wherein the stretchratio of the web introduced at the exit of the drive roll is betweenabout 1.5/1 and about 10/1.
 42. The process as claimed in claim 41,wherein the stretch ratio of the web introduced at the exit of the driveroll is between about 2/1 and about 6/1.
 43. The process as claimed inclaim 1, wherein the web is stretched in several successive steps, thefirst step being conducted by pulling said web at the instant it leavesthe drive roll tangentially: a) at a temperature of between about 40° C.(104° F.) and about 165° C. (329° F.); and b) to between about 20% andabout 80% of the total stretch desired.
 44. The process as claimed inclaim 43, wherein, when the stretching is carried out in two steps, asecond step conducted in a second stretching zone comprising two pairsof rollers, one for forwarding and the other for further stretching theweb, at a temperature ranging between about 40° C. (104° F.) and about165° C. (329° F.) and to about 20% and about 80% of the total stretchdesired.
 45. The process as claimed in claim 43, wherein when thestretching is carried out in three steps, second and third steps arecarried out in separate stretching zones, each comprising two pairs ofrollers, one for forwarding and the other for stretching the web, thestretching temperature in the three steps is between about 40° C. (104°F.) and about 165° C. (329° F.), and the stretching effected: a) in thefirst step, is between about 20% and about 70% of the total stretch; b)in the second step, is between about 15% and about 70% of the totalstretch; and c) in the third step, is between about 15% and about 70% ofthe total stretch.
 46. The process as claimed in claim 43, wherein thepartially stretched film coming from the first stretching step undergoesrelaxation at a temperature of between about 125° C. (257° F.) and about20° C. (68° F.) and is then reheated to a temperature of between 40° C.(107° F.) and 165° C. (329° F.) before entering the second stretchingstep.
 47. The process as claimed in claim 43, wherein the stretched filmundergoes a poststretching relaxation-cooling treatment.
 48. The processas claimed in claim 46, after which the stretched film undergoes apoststretching relaxation-cooling treatment, wherein the stretched filmundergoes at least one embossing step conducted during at least one ofthe stretching, relaxation, relaxation and reheating and relaxation andrecooling steps using, as embossing means, the surface of at least oneof the rollers used for the stretching, relaxation, relaxation andreheating, relaxation and cooling steps said surface being provided withan embossing pattern.
 49. The process as claimed in claim 1, wherein theembossing is carried out in succession on the web and at least once onthe film.
 50. The process as claimed in claim 1, wherein thelongitudinally stretched web is stretched transversely with a stretchratio of between about 1.5/1 and about 10/1.
 51. The process as claimedin claim 50, wherein the longitudinally stretched web is stretchedtransversely with a stretch ratio of between about 1.5/1 and about 4/1.52. The process as claimed in claim 1, wherein the stretched film isstored on a suitable support, at a rate of from about 0.5% to about 25%less than that of the preceding stretching operation allowing forrelaxation.